Expert Q&A

Saturated, Unsaturated & Trans Fat: What Is the Difference?

I'm confused about all the different types of fats on food labels. It seems pretty important, given the FDA's recent ruling about trans fats. Can you explain what the difference is between all the different kinds of fats?

Just when we thought unsaturated fat was okay, the Food and Drug Administration issued a warning about trans fat. Apparently, it is such an important health warning that they are requiring food producers to label how much trans fat is contained in particular foods. To understand the difference between trans fats and all the other fats, it is necessary to understand how fats actually work behind the scenes, on the molecular level.

The word fat refers to a substance known in chemistry as glyceride. Glycerides have a backbone of glycerol. As many as 3 fatty acids can attach to the glycerol backbone. Because fats usually have 3 fatty acids, when they attach to the glycerol they are called triglycerides.

Fatty acids are long molecules made up of carbon atoms with hydrogen atoms attached to them and oxygen at one end. There are 4 types of fatty acids:

  • Saturated - carbon atoms have all the hydrogen they can hold
  • Unsaturated - carbon atoms can hold more hydrogen
  • Mono-unsaturated - only 1 carbon still has room for hydrogen
  • Polyunsaturated - many carbons have room for hydrogen.

Unsaturated fats are typically liquid, like oils. Saturated fats tend to be in solid form, like Crisco and butter. Foods, like cookies, tend to be flat when made with oil; whereas when made with butter or Crisco, they are thicker and fluffier. Also, oils tend to separate from foods, while fats stay in place. Food manufacturers have learned that by adding hydrogen to unsaturated fats (oils), they can change their properties, resulting in much more satisfactory cooking results. Hydrogenated oil in food improves the texture, but it also means more saturated fat.

Artificially hydrogenated fats tend to look different than naturally saturated fats. If you look at them under a microscope, you would see that artificially hydrogenated fats are twisted in a different direction than their natural siblings. The 2 adjacent carbons with only 1 hydrogen have the hydrogen on opposite sides. Chemists call this the trans configuration. It turns out that trans fats confuse our biochemistry, causing arteries to harden quicker, and increasing inflammation.

Read the nutrition facts on food labels. If the food has less than half a gram of trans fat, it can be called trans fat free. However, just because a food is trans fat free or high in polyunsaturated fat does not necessarily mean it is good for those watching their weight. All fats have 9 calories per gram compared to 4 calories per gram for carbs and protein. Those brownies may be trans fat free, but they still have calories. So, try to avoid trans fats and consume vegetables, fruits, and other foods with naturally unsaturated fats.

John Messmer, MD
Contributing Expert

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